In the technical arts, there are many situations when it is desired to determine the size of an object in a photograph or other image. This may occur during photogrammetry, photo-intelligence, on maps and during analysis of aerial or space based images, or in connection with medical images, as for example, images produced by X-ray, Cat Scan, Ultra-Sound, PET Scan Tomographic or other apparatus. The image may be on film or a film print or on a projection screen or on an electronic display such as a CRT or other media. As used herein, the word "film" is intended to include all display media and the word "image" is intended to refer to any type of displayed information, as for example but not limited to those types of images mentioned above.
A number of means and methods have been developed in the prior an for determining the size or placement or displacement of objects in images. For example, U.S. Pat. No. 2,557,428 to Grostic describes an instrument for determining displacements in X-ray images, especially lateral displacements of spinal vertebra. In U.S. Pat. No. 2,819,526, Brown, Jr., described a calculator for use with X-ray images for determining object size. In U.S. Pat. No. 4,131,998, Spears described a tumor growth measurement device for use with X-ray images. In U.S. Pat. No. 4,630,375, Spolyar describes an apparatus for gauging and determining spatial coordinates for a source of radiation to be employed in obtaining a radiograph of a patient, especially in connection with dental images. In U.S. Pat. No. 4,481,719, Grover describes an apparatus and method for measuring the displacement of the arms of a drafting machine and in U.S. Pat. No. 4,974,164, Lewis et at., describe a digital measuring and proportioning instrument comprising a hand-held microcomputer based ruler-like measuring and calculating instrument that is particularly adapted for measuring the sizes of objects in the field of graphic arts. In U.S. Pat. No. 5,170,570, Mays, Jr., describes a hand, finger and joint measuring gauge comprising a device having V-shaped notches therein with an associated scale showing the widths of the notches at different locations therein.
While the foregoing and other prior an devices and methods have proved useful, they suffer from a number of disadvantages well known in the art. For example, some require that the magnification ratio be known, or that the source to object to film-plane distances be known. Others are complex to use or costly or have an inadequate measurement range.
In cases where the exact magnification ratio of the image is not known, it must be determined from the image itself. With many of the prior art methods this is not easily accomplished. One familiar method is to measure the dimension in the image of an object which is of known size and then use this information to determine the actual size of other objects visible in the same image. For example, if a medical X-ray image shows the presence of a catheter known to be 2 mm in actual diameter, but appearing in the image to be only 1.5 mm in diameter, then by simple arithmetic, one knows that the adjacent blood vessel which has an apparent size in the image of 2.5 mm, is actually (2.0/1.5).times.(2.5)=3.3 mm. The scaling may be performed by hand or mental calculation, or using a slide rule or calculator, or by using look-up tables.
This practice of scaling from objects of known size to determine the size of other objects in an image of unknown magnification, is well known in the art. However, the prior art practice of using, for example, a ruler and calculator to perform the measurement and scaling is highly prone to errors. In the medical imaging field, measurement errors can have life threatening consequences. For example, suppose a major blood vessel appearing on a medical image is erroneously determined to be larger than it actually is. Based on that erroneous measurement an operator might mistakenly inform the medical practitioner that a catheter of a certain size would fit in the blood vessel. But, because a mistake has been made in measuring the apparent blood vessel size on the medical image, when the catheter is inserted it could rupture the blood vessel wall, possibly causing severe internal bleeding with potentially life threatening consequences.
Thus, there is a continuing need for an improved means and method for measuring the size of objects in images of a known or unknown magnification and for determining the magnification. While this need is particularly acute in the context of medical imaging because of the potential impact of errors on human life, it is also important to other fields of endeavor where accuracy and ease of measuring the size of various objects and/or the magnification of an image are also important.